The present invention pertains to air-oxygen-fuel combustion processes.
A large majority of combustion processes use air as an oxidizer to combust a fuel such as natural gas, fuel oil, propane, waste oils, and other hydrocarbons and the like. Performance of many air-fuel combustion processes can be improved by enriching the combustion air with oxygen. Enrichment of the combustion air increases both the flame temperature and the thermal efficiency while the flue gas volume decreases as the oxygen concentration in the oxidizer increases. The cost of using high purity oxygen for enrichment can be offset by gains in productivity from enhanced combustion. Low level enrichment of up to 35% total oxygen content in the oxidizer can generally be applied to existing air-fuel systems with only a few modifications to the system.
It is well known that using oxygen to enhance combustion has many benefits which include increasing productivity and thermal efficiency which are both of interest in many of the high temperature heating and melting processes used in industry. However, the cost of completely replacing the combustion air with high purity oxygen is often not cost-justified, and may not be required or desirable. In those cases, it is better to use an intermediate oxygen composition which is a combination of air with high purity oxygen. It has been demonstrated that there is initially a rapid rise in the benefit as the oxygen content and the oxidizer increases up to about 60%. Above 60%, the benefits still increase with the oxygen content in the oxidizer however, at a much lower rate. Therefore, this is in effect a case of economic diminishing returns.
Air Products and Chemicals, Inc. of Allentown, Pennsylvania has introduced, under the brand name EZ-Fire, the use of both air and oxygen in combustion processes to maximize the benefit to cost ratio. However, even with these burners the problem is still one of NOx generation. NOx rises rapidly to a peak in the middle range of oxidizer compositions, and then decreases to zero when pure methane is combusted with oxidizers ranging in composition from air through mixtures of air and pure oxygen to pure oxygen since no nitrogen is present when pure oxygen is the oxidizer. The challenge to the operator when using intermediate oxygen compositions is to control the NOx emissions. Air-fuel burner manufacturers have designed new, low NOx burners which incorporate many of the known techniques for minimizing NOx formation including fuel or furnace gas recirculation, oxidizer or fuel staging, pulse combustion, and controlled delayed mixing. However, in many cases there is a reduction in thermal efficiency and productivity. There are many methods for reducing NOx after it has been formed. These methods are usually referred to as post-treatment however, this type of NOx reduction equipment is strictly an add-on cost, with no process benefit.
Solutions to the problem include a burner such as offered for sale by Air Products and Chemicals, Inc. and as disclosed in U.S. Pat. No. 5,308,239, which will increase productivity and thermal efficiency while minimizing NOx by enabling the user to retrofit the burner to existing dual-fuel air-fuel burners.
An improvement to the burner shown in the '239 patent is offered for sale by Air Products and Chemicals, Inc. and is disclosed and claimed in U.S. Pat. No. 5,611,683.
Another technique is disclosed and claimed in U.S. Pat. No. 4,797,087 wherein a core of oxygen is surrounded by a sheath of one or more fuels which are then surrounded by air. Another version of this technology has an inner core of fuel surrounded by oxygen which is surrounded by an auxiliary fuel which is then surrounded by air. The main objective of this technology is to create an inner, fuel rich pyrolizing zone to produce soot and thus a luminous flame. The outer zone is fuel lean to burn out the soot and any remaining fuel. This type of a flame has two or more reaction zones depending upon which embodiment is employed. The primary embodiment has two reaction zones; one reaction zone between the inner oxygen and fuel and one between the fuel and air to produce a highly luminous flame but at the same time also high NOx contents.
U.S. Pat. No. 5,454,712 discloses and claims an air-oxy-fuel burner where the fuel and oxygen are in the core but the air is swirled around this inner flame zone to intensify mixing. As will be detailed below this, in principal is diametrically opposed to the goal of the present invention.
U.S. Pat. No. 5,217,363 discloses and claims an air-oxy-fuel burner with fuel in the middle surrounded by oxygen, which is then surrounded by air. The core of the '363 burner has a spark ignitor so that the fuel passage is an annulus instead of being circular as in all of the Air Products burners. The '363 burners have moveable passages which are used to change the deflection angles and velocities of each gas while the prior Air Products devices all have straight passages. Lastly, the air in burner of the '363 patent is primarily used for cooling, not combustion whereas the Air Products devices employ air as an integral part of the combustion process.